The invention relates to a process for producing an electrode for a fused carbonate fuel cell, an electrode made according to the process, and a fused carbonate fuel cell with an electrode produced according to this process.
The production of cathodes for carbonate melt fuel cells from lithium cobaltite (LiCoO.sub.2) is known. For making such cathodes, lithium cobaltite powder is mixed with a binder. A dispersant can be added to the binder. A foil is made from the mixture which is divided into plates. The plates are sintered at high temperatures in an air-carbon dioxide atmosphere. Lithium cobaltite is made by reacting cobalt with lithium compounds (EP 0 473 236 A2).
Making lithium cobaltite by reacting cobalt oxide (iron oxide) with lithium hydroxide vapor as a powder in a high-temperature reaction is also known. This powder is made into brittle electrode plates with small dimensions by a ceramic sintering process (JP 0636, 770).
Finally, making a lithium cobaltite layer from a ductile cobalt layer whose pores are filled with lithium carbonate is known. Conversion to the lithium cobaltite layer is preferably done after combination with a matrix layer and an anode layer and after installation together with current collector plates in a cell holder of a fuel cell during the start-up phase of the fuel cell. The structure of the lithium cobaltite electrode plate made in this way corresponds to the structure of the original porous cobalt electrode plate which has a relatively high polarization resistance (DE 43 03 136 C1).
The invention is based on the problem of creating a process for producing a porous lithium cobaltite electrode plate with a large internal surface area and a low polarization resistance and producing an electrode plate made according to the process.
The problem is solved for the process according to the invention by mixing cobalt metal powder and lithium carbonate powder with each other homogeneously then producing foils from the mixture and plates from the foils. The plates are sintered into porous electrode precursor plates are then exposed electrode precursor plates to an air flow for several hours at a temperature of between 400.degree. C. and 488.degree. C., until the electrode precursor plates have been converted into lithium cobaltite electrode plates with extremely large internal surface areas. In the process according to the invention, a lithium cobaltite formation reaction that determines the structure takes several hours. Initially, cobalt in the porous cobalt/lithium carbonate precursor electrode plate is oxidized in the atmosphere of air. Then, lithium cobaltite and lithium oxide are formed at the points where cobalt oxide contacts lithium carbonate, releasing carbon dioxide which is carried away with the air current. Because of its high vapor pressure, lithium oxide changes to the gas phase in which it reacts with cobalt oxide that has not contacted lithium carbonate to form lithium cobaltite.
While the lithium cobalt is being formed, it is preferable to keep the temperature at 420.degree. C. to 480.degree. C. It has been shown that in this temperature range the above-described reactions take place under favorable conditions, influenced by atmospheric oxygen.
In particular, the quantity of air admitted and the air flowrate are adjusted such that the carbon dioxide level in the air is no higher than approximately 1% and the air is allowed to act for approximately 10 hours. Under these conditions, an electrode consisting of lithium cobaltite with a very large internal surface area of 2 to 6 m.sup.2 /g is obtained, that no longer contains any lithium carbonate.
The reactions that take place in the above-described process during the various phases are described in detail below:
Mechanism of Solid-Gas Reaction (400.degree. C.-488.degree. C.)
Oxidation of cobalt:
Co+2/3O.sub.2.fwdarw.1/3Co.sub.3 O.sub.4 PA1 1/3Co.sub.3 O.sub.4 +Li.sub.2 CO.sub.3 +1/12O.sub.2.fwdarw.LiCoO.sub.2 +CO.sub.2 +1/2Li.sub.2 O.sub.(solid) PA1 Li.sub.2 O.sub.(fest).fwdarw.Li.sub.2 O.sub.(gas) PA1 Li.sub.2 O.sub.(gas) (reaction location 1).fwdarw.diffusion.fwdarw.Li.sub.2 O.sub.(gas) (reaction location 2) PA1 a) 1/3Co.sub.3 O.sub.4 +Li.sub.2 O.sub.(gas) +1/12O.sub.2.fwdarw.LiCoO.sub.2 PA1 Li.sub.2 CO.sub.3 +H.sub.2 O.sub.(g).fwdarw.2LiOH.sub.(g) +CO.sub.2 PA1 Li.sub.2 O.sub.(s,g) +H.sub.2 O.sub.(g).fwdarw.2LiOH.sub.(g) ##STR1## PA1 1/3Co.sub.3 O.sub.4 +LiOH.sub.(g) +1/12O.sub.2.fwdarw.LiCoO.sub.2 +1/2H.sub.2 O.sub.(g)
Solid reaction at cobalt oxide/lithium carbonate contact points:
Gas diffusion of Li.sub.2 O.sub.(gas) :
Gas-solid reaction at reaction location 2:
When the carbon dioxide component of the air is small, not exceeding a value of 1%, an electrode consisting of lithium cobaltite with a very large internal surface area of 2-6 m.sup.2 /g is obtained, which no longer has any Li.sub.2 CO.sub.3 after ten hours. The structure formed during this combined oxidation/activation process is retained when the electrode is used in a carbonate melt fuel cell.
Li.sub.2 O diffusion into oxidized CO particles that have not contacted Li.sub.2 CO.sub.3 particles is not hindered by increasing the CO.sub.2 level of the air since Li.sub.2 CO.sub.3 forms from the Li.sub.2 O and CO.sub.2 once again. Consequently the rate of LiCoO.sub.2 formation decreases with a rising CO.sub.2 level in the air and the desired fine structure with a large internal surface area cannot form.
When moist air is used with vapor levels of over 2%, the activation process for forming a larger internal surface area of over 2 m.sup.2 /g can also be carried out with CO.sub.2 levels of over 1% in the activation gas atmosphere. In this case, LiCoO.sub.2 is formed by reacting oxidized cobalt with lithium hydroxide according to the following mechanism.
LiCoO.sub.2 formation in the presence of water vapor:
When the vapor level is raised to over 2%, the reaction is not accelerated further.
The process according to the invention can be carried out after the electrode precursor plates have been placed in an oven under the conditions described above, removing the lithium cobaltite electrodes after the oven has cooled and assembling them with a matrix layer saturated with the molten electrolyte and an anode as well as with current collectors to form a fuel cell.
It is also favorable to combine the electrode precursor plate in question with a matrix layer filled with fused carbonate into a layer arrangement corresponding to the fuel cell and then to build it into a fuel cell together with the latter, and to carry out the process according to the invention after installation in the fuel cell. Under the conditions of the process according to the invention, the lithium cobaltite cathode is formed during a fuel cell start-up procedure. The lithium cobaltite can also be produced as a thin, adhesive layer on a porous nickel substrate, which thus becomes oxidized.